The exhaust gas from motor vehicles which use gasoline as the fuel contains harmful components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). It is required to purify each of the harmful components by using a catalyst so that the hydrocarbons (HC) are converted into water and carbon dioxide by oxidation, the carbon monoxide (CO) is converted into carbon dioxide by oxidation, and the nitrogen oxides (NOx) are converted into nitrogen by reduction.
As a catalyst (hereinafter referred to as the “exhaust gas purifying catalyst”) for treating such an exhaust gas, a three way catalyst (TWC) capable of redoxing CO, HC, and NOx is used.
As such a three way catalyst, those that are obtained by supporting a precious metal such as platinum (Pt), palladium (Pd), or rhodium (Rh) on a refractory oxide porous material having a high specific surface area, for example, an alumina porous material having a high specific surface area and supporting this on a substrate, for example, a monolithic substrate which is made of a refractory ceramic or a metal and has a honeycomb structure or those that are obtained by supporting this on refractory particles are known.
In recent years, an invention has been disclosed which is intended to use yttrium manganite composite oxide (YMO) as an oxidation catalyst for purifying exhaust gas although it is used as an oxidation catalyst for diesel engine.
For example, an oxidation catalyst for purifying exhaust gas that is composed of a composite metal oxide represented by a general formula Y1-xAgxMnO3 (0.01≤x≤0.15) or a general formula Y1-xAgxMn1-yAyO3 (A is one kind of metal selected from the group consisting of Ti, Nb, Ta, and Ru. 0.005≤y≤0.2) is disclosed in Patent Document 1 (JP 2008-100184 A).
An oxidation catalyst composed of a mixture of any one kind of composite metal oxide among the composite metal oxides that are represented by a general formula Y1-xAgxMn1-yAyO3, in which A is one kind of metal selected from the group consisting of Ti, Ce, and Ru, 0.01≤x≤0.30, and 0.005≤y≤0.30 and zirconium oxide is disclosed in Patent Document 2 (JP 2009-279579 A).
An oxidation catalyst composed of any one kind of composite metal oxide represented by a chemical formula YMnO3, Y1-xAgxMnO3 (0.01≤x≤0.30), Y1-xAgxMn1-yAyO3 (0.01≤x≤0.30 and 0.005≤y≤0.30, and A is any one kind of metal of Ti, Nb, Ta, Ru, Ce, or Fe) is disclosed in Patent Document 3 (JP 2010-284583 A.).
An oxidation catalyst for purifying exhaust gas which contains a composite metal oxide represented by a general formula Y1-xAgxMn1-yFeyO3 (0.01≤x≤0.30 and 0.01≤y≤0.50) is disclosed in Patent Document 4 (JP 2010-284584 A).
A catalyst for purifying exhaust gas which contains a double oxide in which the site A contains Y, the site B contains Mn, and the composition ratio B/A of the site B to the site A is greater than 2 and which is composed of a crystal taking a structure DyMn2O5 is disclosed in Patent Document 5 (JP 2013-233541 A).
A catalyst for purifying exhaust gas which contains a catalyst support composed of a ceramic or a metallic material, a double oxide Y1-xAxMn2-zBzO5 (in the formula, A is La, Sr, Ce, Ba, Ca, Sc, Ho, Er, Tm, Yb, Lu, or Bi, B is Co, Fe, Ni, Cr, Mg, Ti, Nb, Ta, Cu, or Ru, 0.5≥x≥0, and 1≥z≥0) that is supported on the catalyst support, and at least one kind of atom that is selected from the group consisting of Ag, Pt, Au, Pd, Rh, Cu, and Mn and supported on the double oxide Y1-xAxMn2-zBzO5 is disclosed in Patent Document 6 (WO 2012/093599 A) as a catalyst for exhaust gas which exhibits excellent oxygen occluding properties and is used to purify the exhaust gas discharged from an internal combustion engine of a motor vehicle or the like.
In addition, a catalyst for purifying exhaust gas that is composed of a tetragonal composite oxide represented by a general formula. A2BO4 (in the general formula, A represents at least one kind selected from the group consisting of Ca, Sr, and Ba, and B represents at least one kind selected from the group consisting of Mn, Fe, Ti, Sn, and V), a refractory inorganic oxide in which at least the surface layer is composed of MgAl2O4, and a precious metal component that is present in the tetragonal composite oxide in the form of a solid solution or supported on the tetragonal composite oxide or the refractory inorganic oxide is disclosed in Patent Document 7 (JP 2006-26554 A) as a catalyst composition for purifying exhaust gas which contains particles containing a manganese-containing composite oxide.
An exhaust gas purifying catalyst having a support formed of an oxide, a great number of palladium oxide (PdO) particles which are supported on the support and have an average particle size of 1 nm or more and 50 nm or less, two or more kinds of composite oxides selected from Mn, Fe, Co, or Ni as the promotor particles, a great number of rare earth oxide (LnOx) particles which are in contact with the palladium oxide (PdO) particles and supported on the support and have an average particle size of from 11 nm to 50 nm is disclosed in Patent Document 8 (JP 2007-105633 A).